This invention relates to electronic surveillance systems, and more in particular to an improved marker for use in such systems.
Electronic surveillance systems of the type to which the present invention is directed, are generally employed to detect the presence of a magnetic marker in a magnetic field. Such systems thus include a device for generating a magnetic field, and a receiver for detecting variations in the field resulting from passing of a marker, generally carried by an article, through the field.
Such a system is disclosed, for example, in U.S. Pat. No. 4,710,752, Cordery. In the system disclosed in this reference, the ability of the system to detect the presence of a marker, in the presence of noise, is enhanced by forming the magnetic field of more than one frequency component.
A magnetic marker "switches", to provide a detectable "output", when the externally applied field passe the coercive field of the ferromagnetic marker. At this time, if the marker is biased, then the phase of the externally applied field at which the marker switches is changed.
Soft magnetic markers for electronic surveillance systems are also disclosed, for example, in U.S. Pat. No. 4,686,154, wherein a marker is comprised of two strips of soft magnetic material having different coercivities but equal magnetic saturation levels.